That's the whole reason for its success: mythic+ is accessible and puggable, especially across realms. With respect to threat, it might not even be an issue if Blizz is successful in its endeavor to slow down combat (again). The problem comes when there are DPS classes that have tons of burst, controlled or not. I can remember the first half of Legion, especially on Skittish weeks, where you could have Fury warriors going splat in their openers. Delaying helped, but it's not likely fun going back to the "wait for 5 sunders" mentality either, especially if the fault is class design and not the player. The threat change won't affect my ability to tank stuff, but it will have some effect on the average tank. If the change is really meant for really high level keys, Blizz could've just had another affix apply to 10/15 keys, where tank threat drops... again, it's assuming the change is only meant for high level keys, which it probably is not.
If I recall, the whole reason for the really high threat threat was that Blizz wanted tanks to focus on survival and "tank stuff" when it comes to mob management and positioning (at least in Legion with respect to at least Taunt, iirc). Sort of a fire-and-forget threat model to help out tanks of lesser skill deal with all the jobs of being a tank. Not saying they're right or wrong in the matter, that's just how it was described in a blue post a while back.